"Wayne" wrote in
:



"W5DXP" wrote in message
news:1fe694d0-ff12-48dd-9abe-
.
..

On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
-
Thanks for the comments. When I used EZNEC I didn't know what to use
for ground, so I used Real/Minimec. I'm showing about 18-j142 as the
feedpoint impedance at 21.1 Mhz, which is not totally out of the
ballpark with your number.

At any rate, I have bought enough telescoping tubing sections to build
about a 20 foot vertical. Final height is subject to approval by "the
Admiral". Fabrication of a more substantial base support will be
necessary.

In the meantime, this AM I replaced the 8.6 ft whip with about 13 feet
of telescoping tubing, using the same mobile whip mount clamped to
conduit. Resonance is around
18.7 MHz and it loads up nicely on 18 through 10, with 10 meters
having the highest calculated swr of 17.

With the antenna removed, the feedline shows a 1/4 wave null at
4.33MHz. This gives an estimated 1/2 wave electrical length of 57 feet
at 8.66 MHz, or with 0.66 VF a physical length of 37.6 ft. That seems
about right.

I'll get a Smith chart and see where the existing feedline moves the
parameters. Per your comments I'll try to avoid low r values.

Wish the bands would open back up like they were in Oct and Nov.
-
Wayne
W5GIE


So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to lose
70% of the power out of the ATU as heat. Of course, it is likely that
the power out of the ATU is somewhat less than the transmitter output as
the ATU load is some 7+j10, relatively low R (but typical ATUs tend to
be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Owen

On Jan 8, 10:21*pm, Owen Duffy wrote:
Did the Smith chart reveal any of that?

There are loss scales at the bottom of the Smith chart that allow
losses to be included. That's why I mentioned "tuner efficiency"
and "considerable feedline losses" in my posting.
--
73, Cecil, w5dxp.com

Owen Duffy wrote in
:

....
So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to
lose 70% of the power out of the ATU as heat. Of course, it is likely
that the power out of the ATU is somewhat less than the transmitter
output as the ATU load is some 7+j10, relatively low R (but typical
ATUs tend to be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Well, subject to how carefully you plotted the data, you might get a
reasonable answer for the line input impedance (as seen by the ATU), but
the Smith chart will not directly read the line loss with a mismatched
losd.

With a lot of care in plotting, you might scale off the values of G at
line input and output, and find the ratio Vload/Vin from the chart to
calculate the ratio of Pload/Pin and so find the loss under mismatched
load.

The line loss in this case is just over 5dB, more than 10 times the
0.5dB loss you would observe with a matched load.

An exercise for the reader is to use a Smith chart to find the loss
under the mismatched load with RG213 where the matched line loss is
about 0.31dB.

Owen

PS: if you had in mind using the handy dandy "Additional loss due to
VSWR" graph in the ARRL, the line input VSWR is 7.7, load VSWR is 23.7.
If you get the same loss figure are mentioned above, it is by accident
of the scenario rather than soundness of the method.

On Jan 9, 3:12*pm, Owen Duffy wrote:
Well, subject to how carefully you plotted the data, you might get a
reasonable answer for the line input impedance (as seen by the ATU), but
the Smith chart will not directly read the line loss with a mismatched
losd.

Owen, I respect your obvious technical ability and the only time I
ever disagree with you is when you go off the deep end and accuse me
of saying things that I never said and/or try to pick a fight like you
are trying to do on this thread. What in the world do you expect to
gain from such antisocial behavior?
--
73, Cecil, w5dxp.com

"Owen Duffy" wrote in message
...

"Wayne" wrote in
:



"W5DXP" wrote in message
news:1fe694d0-ff12-48dd-9abe-
.
..

On Jan 2, 3:39 am, "Wayne" wrote:
Still having fun playing with my 8.5 foot whip on the metal patio
roof, with coax feedline to an antenna tuner in the shack.
I was unable to get a match on 15 meters with the antenna tuner.

EZNEC sez the feedpoint impedance is around 17-j175 ohms with an SWR
of ~40:1 at the antenna feedpoint. Minimum impedance ~1.25 ohms,
depending on feedline length. Not many tuners will match 1.25 ohms
with reasonable efficiency. In this case, a feedline length of an
integer number of wavelengths (N*30') plus 13 feet would probably
allow for a reasonable tuner match for VF=0.66 coax if you can
tolerate the considerable feedline losses.
--
73, Cecil, w5dxp.com
-
Thanks for the comments. When I used EZNEC I didn't know what to use
for ground, so I used Real/Minimec. I'm showing about 18-j142 as the
feedpoint impedance at 21.1 Mhz, which is not totally out of the
ballpark with your number.

snip
***********

Update. Current configuration is a 14.6 ft element fed with 40 ft RG-8.

Wayne
W5GIE


So, if the feed point impedance was 18-j142, and you used 38' of RG58
(for example, I could not find that info), then you would expect to lose
70% of the power out of the ATU as heat. Of course, it is likely that
the power out of the ATU is somewhat less than the transmitter output as
the ATU load is some 7+j10, relatively low R (but typical ATUs tend to
be more efficient on the higher bands).

Did the Smith chart reveal any of that?

Owen
-
I think that losses were discussed on a previous iteration of the thread,
and I have no quarrel with the loss numbers.

Many years ago, I might have been able to somehow deduce cable losses from a
Smith chart, but nowadays I just use the published curves.

The current configuration (14.6 ft vertical) is promising for 14-28 MHz
operation, as three of the bands have SWRs of less than about 10:1, and the
other two are below about 20:1. The antenna is not checked out on all of
those bands, but it does an acceptable job on 12 and 10 meters where the SWR
is highest. That statement should not be construed as meaning that it is
the final configuration.

On Jan 9, 6:29*pm, W5DXP wrote:
Owen, I respect your obvious technical ability ...

Owen, I apologize for my previous bad mood outburst.
--
73, Cecil, w5dxp.com

"Wayne" wrote in
:

Many years ago, I might have been able to somehow deduce cable losses
from a Smith chart, but nowadays I just use the published curves.

An email correspondent suggested the radially scaled "SW LOSS COEFF"
gives a direct reading loss under mismatch from the chart.

There are assumptions that underly that scale, pre-requisites if you
like for it to be accurate. They are explained in Smith's book: "If a
waveguide is one or more wavelengths long, the average increase in
dissipative loss due to standing waves in a region extending plus or
minus one-half wavelength from the point of observation may be expressed
as a coefficient or factor of the one-way transmission loss per unit
length".

So, it does not apply to a feed line of less than a full wave, or any
residual of less than a full a wave after working out the additional
loss in each of the full wave segments of a line. It is tedious and
accurate only for lines of integral full waves length.

The potential error applying it to arbitrary line length ranges from
insignificant to huge.

The graphs in the ARRL suffer the same problem, and IIRC they do not
state the assumptions.

The Smith chart solution I described in my earlier post works for
arbitrary line lengths, and properly reveals the cases where loss under
standing waves is less than matched line loss. TLLC
(http://www.vk1od.net/calc/tl/tllc.php) also calculates the loss
correctly for arbitrary line lengths.

Owen

"Owen Duffy" wrote in message
...

"Wayne" wrote in
:

Many years ago, I might have been able to somehow deduce cable losses
from a Smith chart, but nowadays I just use the published curves.

***

The Smith chart solution I described in my earlier post works for
arbitrary line lengths, and properly reveals the cases where loss under
standing waves is less than matched line loss. TLLC
(http://www.vk1od.net/calc/tl/tllc.php) also calculates the loss
correctly for arbitrary line lengths.

Owen
-
Great link, Owen. I have it bookmarked and will do some playing around with
it.
Wayne
W5GIE